I was thinking about 'River Anacostia' yesterday and idly Googling came across a reference to an art presentation put on in April 2013 by an artist and a teacher at the State University of New York, Potsdam, entitled 'Wade in the Water: the River Anacostia Project' and described as a 'creative transdisciplinary journey'. There is some very limited information here: https://pluto.potsdam.edu/academicfestwiki/index.php/Wade_in_the_Water:_The_Anacostia_River_Project,_A_Creative_Transdisciplinary_Journey. New York is a long way from Washington, but this was around the time Polly and Seamus Murphy were making their first exploratory visit to DC, and I think the title is too striking to be a coincidence. This must be the route by which she connected the slave spiritual with the river landscape, though how it came to her attention is anyone's guess.